1. Field of the Invention
The present invention relates to a differential amplifier circuit whose transient response characteristic is improved, and particularly to a voltage controlling differential amplifier circuit constituting a direct-current (DC) power supply device such as a series regulator, and a technique which is effectively applied to the series regulator which uses the differential amplifier circuit.
2. Description of Related Art
A series regulator that outputs a constant DC voltage by controlling a resistance value of a control transistor connected between a voltage input terminal and a voltage output terminal is in widespread use as a DC power supply device. In the series regulator, a differential amplifier circuit illustrated in FIG. 4 is used as a circuit that generates a voltage applied to a control terminal (a gate terminal or a base terminal) of the control transistor according to a feedback voltage. A differential amplifier circuit 10 illustrated in FIG. 4 includes a differential input stage 11 and an output stage 12. The differential input stage 11 includes a pair of differential metal-oxide semiconductor (MOS) transistors (insulated gate field effect transistors) Mp1 and Mp2 whose sources are commonly connected, load MOS transistors Mn1 and Mn2 that are connected to drains of the differential MOS transistors Mp1 and Mp2, respectively, and a constant-current MOS transistor Mp0 that is connected between the common source of the differential MOS transistors Mp1 and Mp2 and a power supply voltage VDD. The differential input stage 11 is configured as a complementary metal-oxide semiconductor (CMOS) differential amplifier circuit.
The output stage 12 includes a constant-current MOS transistor Mp3 and a MOS transistor Mn3. The gate of the constant-current MOS transistor Mp3 and the gate of the constant-current MOS transistor Mp0 of the differential input stage 11 are commonly connected. The MOS transistor Mn3 is connected in series to the constant-current MOS transistor Mp3 between the power supply voltage VDD and a ground point. The output stage 12 is configured such that the drain of the differential MOS transistor Mp2 of the differential input stage 11 is connected to a gate terminal of the MOS transistor Mn3 and such that a control MOS transistor Mp4 composed of a power MOS transistor is controlled by a voltage at a connection node N1 of the constant-current MOS transistor Mp3 and the MOS transistor Mn3.
A common bias voltage Vb is applied to the gate terminals of the constant-current MOS transistors Mp0 and Mp3, and identical or proportional constant currents are allowed to flow through the constant-current MOS transistors Mp0 and Mp3. A reference voltage Vr is applied to the gate terminal of the differential MOS transistor Mp1 on a non-inverting input side of the differential input stage 11, and a voltage that is divided by voltage dividing resistors R1 and R2 which are connected in series between an output terminal OUT and the ground point is applied as a feedback voltage FB to the gate terminal of the differential MOS transistor Mp2. Thereby, the differential amplifier circuit 10 controls the control MOS transistor Mp4 such that the feedback voltage FB and the reference voltage Vr match with each other.
In the series regulator illustrated in FIG. 4, it is not going to matter much when a load connected to the output terminal OUT is light. However, when a heavy load is connected, the degraded transient response characteristic of the differential amplifier circuit cannot keep the output voltage constant. On the other hand, the transient response characteristic of the differential amplifier circuit illustrated in FIG. 4 depends on currents flowing through the constant-current MOS transistors Mp0 and Mp3 of the differential input stage 11 and the output stage 12, and the transient response characteristic improves with increasing currents flowing through the constant-current MOS transistors Mp0 and Mp3.
Therefore, the voltage controlling differential amplifier circuit of the series regulator is conventionally designed such that the currents flowing through the constant-current MOS transistors Mp0 and Mp3 are increased when the series regulator is used in a heavy-load system. However, when the series regulator is used in the light-load system, unfortunately a consumption current of the differential amplifier circuit is unnecessarily increased which causes degradation of power efficiency.
To solve this problem, Japanese Patent Application Laid-Open No. 2004-240646, for example, discloses a technique of providing a bias current varying circuit that changes a current flowing through a constant-current source according to a load variation. However, the bias current varying circuit disclosed in Japanese Patent Application Laid-Open No. 2004-240646 has a problem in that the power efficiency is decreased. That is because nearly 10 elements are used, and thereby a circuit occupies a large area and the consumption current of the bias current varying circuit flows as a wasted current.